Histogram / RAW conversion comparison

[neuroanatomist]

I'm still thinking that what I'm seeing is more accurate.

I was thinking more on this as I drove into work this morning. I do think the representation I'm seeing (where the DR of a 14-bit image is displayed as being wider than the DR of a 12-bit image) is more accurate than what you're seeing, Johnny. But, I should point out that neither is truly accurate, and so the differences are probably irrelevant in a practical sense. As I stated above,log₂(2^x) = x, so if the scale being used is log₂of pixel luminance, a 14-bit image should have a DR of 14 units. The range of -11 to +3.8 (or so) is ~14.8, nearly a full unit wider than it should be for a 14-bit image; likewise, I see a 12-bit image displayed with a range of 12.8 units.


Regardless of the actual units, DPP is not 'throwing away' any of the dynamic range of the RAW image (until it's converted to an 8-bit format like JPG, that is). So, practically it doesn't really matter if the scale is-11 to +3.8,-9 to +3.8, 0-16,384, -20 to +20, or even if there are no units at all. So long as the pixels are binned across a range (and those bins are sufficiently narrow as to allow meaningful adjustments), the histogram serves its purpose.

[Johnny Rasmussen]

I came across a discussion in the dpreview forum. It seems like we are not the only ones that have been wondering about this. One guy said that the differences seen in the histograms is "canons way of showing us the wider DR of the 14-bit raw-files". He argued that a 14-bit file should be able to show a 2 unit wider DR. I went to Canon Japan, looked at DPP tutorials and they had the same wide DR as your version.


Anyway, you are right, even if the representation of DR differs it does not really matters. I can live with it []


I was thinking about something else, John. You mentioned you did not shoot with HTP on because of increased shadow noise. Is it really that visible? I have not shot with the 7D but it is hard to see any difference in files from the 50D. If I enlarge the image 200% I can see a small difference. Bob Atkins tested the HTP function http://www.bobatkins.com/photography/digital/canon_eos_40D_review_6.html#highlight and particularly looked at the shadows. The link is for the 40D but he has also tested the 50D and the 7D. He said that the differences between these models are insignificant in the shadow region. I know that sites like dpreview criticized the 50D compared to the 40D but I wonder if this is because of the methodology they are using.

[neuroanatomist]

You mentioned you did not shoot with HTP on because of increased shadow noise. Is it really that visible? I have not shot with the 7D but it is hard to see any difference in files from the 50D.

It's rather situationally dependent. I have seen it in some shots, not in others. I suspect it's a combination of factors. Even Bob Atkins stated (from the article you linked), "Looking closely at images shot with Highlight Tone Priority turned on and off, there is an increase in the shadow noise if you look closely enough." As I stated, I don't usually use it. But, I do use it occasionally (in fact, it's one of the settings I put in My Menu) in situations where looking at the scene I expect issues from blown highlights at the exposure I plan to use.


Daniel summarized it nicely in a previous discussion on 'tweener' ISO settings:



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It's a pretty minor effect by itself. Just like HTP is a pretty small effect, and ALO is a pretty small effect. But when you start combining all these small factors, it results in a big factor. For example, ISO 250 by itself may not increase shadow noise enough to notice. And ISO 200+HTP by itself may not either. But combine 250+HTP and the result will be much more noticeable: the shadow noise is as bad as ISO 640. Add ALO into the mix and you can get people wondering why their ISO 250 shot looks like ISO 1600.

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[Johnny Rasmussen]

Agree. I also think it's a combination of factors. I also think HTP is a nice feature but you have to use it when the situation calls for it.

[Daniel Browning]

You mentioned you did not shoot with HTP on because of increased shadow noise. Is it really that visible?

It depends on the ISO. HTP works in the context of a constant display brightness and a constant exposure. In that context, low ISO has more noise than high ISO. In other words, when you keep your exposure the same, and the raw conversion is made to come out with the same brightness, higher ISO has less noise (sometimes a lot less) than low ISO. This is the opposite of how we normally think about ISO because we don't normally keep exposure fixed. In essence, it separates ISO-as-a-camera-setting from ISO-as-an-exposure-index.


So ISO 3200+HTP (which is actually just ISO 1600) has the exact same noise as ISO 3200 without HTP, but ISO 200 has less noise than ISO 200+HTP (which is actually just ISO 100).


The way HTP works is pretty simple. Say you shot a scene at ISO 200 f/4 and developed it with default raw converter settings, and it came out with too much blown highlights. Then you shot it at ISO 100 f/4. Now the highlights are better, but the overall image (midtones, shadows, etc.) are too dark. So you push the midtones and shadows without blowing the highlights (e.g. by using a curve). This makes the increased noise of a lower ISO setting visible (you wouldn't see the difference if you left the overal image darker). That is what HTP does.

[canoli]

This makes the increased noise of a lower ISO setting visible (you wouldn't see the difference if you left the overal image darker). That is what HTP does.

The manuals always say to beware of HTP because it'll increase shadow noise. I guess the authors assume we'll want to open up the shadows (and 1/4 tones) in PP, thereby "increasing" the noise.


Seems like HTP is a mixed bag - gain HL range but then you either live with a darker (and flatter) image or reveal that nasty pattern noise in the shadows...